Hi there,
this could help. I garden in a warmer climate (Z 7, Switzerland), about mid May + 2 weeks is the time to plant tubers around here.

I usually pot up some tubers to give them a head start, helps with slug attacks and really gives earlier flowers. I just use a window sill and a balcony and start them 6 to 4 weeks earlier, moving them outside on sunny days.
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having said that: I reckon it`s most important that the soil is warm enough and not with stagnant water. I got some tubers last years, not sprouted or anything around June. After planting they really took off. A neighbor at the garden planted huge clumps at the same time without dividing them, and they started much later, well go figure.

I start most of my dahlias in pots. I start them in march in 4 inch pots. When they get to big I move them into larger pots. This year I will start over a hundred tubers in the poly house in march. Some will stay in pots all season long. Due to the bad slug problem around here I will never again put tubers direct into the ground. They ( slugs) destroyed over twenty tubers over night in the ground.

I also start all of mine in pots to get a jump on the season. I dont plant them out until the end of May, so I usually pot them up mid to late March. It is still usually the end of July before I start getting regular blooms, but they are worth waiting for.

I start mine in March also. I checked back with my post here and found I had my first dinner plate dahlia on July 3 this year. This is a new record for me due to an early spring. We tend to be a bit warmer than mandolls area.

Thanks all ... so if you start them in March, besides in a poly house ... where do you have them growing? Though now that I think about it, I do have a poly house I could use ... but we're much colder than LINY ...

I have a space that used to be an attached garage , that I converted to a heated studio/workroom. I have shelves with fluorescent lights, though I have had pretty good luck starting the dahlias on southern window sills. Even in March a poly house wouldnt be warm enough where I am.

Like Mr. Viking I have seeds started now of various things to help my psyche get through the winter. It helps a lot.

I am also going to start tubers in cowpots this year. That way I can start with a 3 inch pot and than place it in a 5 inch pot without harming roots.
What I want to know is what do you put in big planters to take up some of the room in the bottem? I don't want to be shoving dirt into ths huge pots?
Viking, Are you sure you don't have voles if the tubers were eaten? The slugs stay on top of the ground.

I used a couple of large planters last year for annuals, and filled the bottom third of them with styrofoam peanuts that I had on hand. It lightened the load considerably. I think I read about that somewhere on GW.

The tubers were not eaten they rotted after the shoots were eaten off. I guess I should have explained better. One of the tubers was in a planter over three feet off the ground. I had sprayed at 11 o'clock at night. When I got up in the morning the shoot was gone.